Infrared detectors have become ubiquitous in the security industry, due to the several advantages these detectors offer over competing systems. In particular, because the energy being detected is emitted heat rather than reflected light, such detectors are suitable for both daytime and nighttime operations. Also, because infrared wavelengths are much longer than visible wavelengths of light, infrared detectors can detect through dust, smoke, clouds, haze, and light rain. Infrared detectors are also highly sensitive, and are generally capable of detecting temperature variations of a fraction of a degree centigrade. In addition, because they operate outside of the visible region of the spectrum, infrared detectors are completely passive and non-intrusive.
Passive infrared (PIR) detectors are a type of infrared detector commonly used in security systems. PIR detectors commonly consist of a housing containing optics to focus thermal infrared energy, a pyroelectric detector onto which the optics focus, and circuitry that can amplify and process the electrical signal from the pyroelectric detector. A simple configuration is a single cylindrical lens in front of a dual-element pyroelectric detector. This configuration leads to a broad, double vertical barrier, with the width of the barrier dependent on the width and spacing of the elements in the detector and on the focal length of the lens.
While this approach can address security against, for instance, attempts to enter a window or to cross a barrier in front of an artwork, it does not address nuisance alarms associated with such security requirements. For example, in some installations of motion detectors, the detector is required to sense motion across a window, storefront or other opening which may be situated adjacent to a busy sidewalk or street. In such installations, if the angular range covered by the detector (that is, the angle of coverage measured perpendicularly to a major plane of coverage) is overly broad, the detector may sense unintended targets, such as innocent passersby or normal street traffic. Even if the coverage area of the detector is tilted toward the window in order to reduce the portion of the coverage area overlapping the public areas, reflections from the window glass of thermal energy from passersby or traffic can be detected, and nuisance alarms may result.
In order to minimize such nuisance alarms, detectors in such installations can be configured with a very long focal length. Since the angular range covered by the detector decreases with increasing focal length (the tangent of the angle of coverage is given by dividing the dimensions of the pyroelectric detector by the focal length of the lens), this approach causes the image of the detector elements to be very narrow, and hence establishes a very narrow sheet of protection in front of the opening to be protected. However, the use of a long focal length also means that the image of an intruder close to the lens will not be focused on the pyroelectric detector. Hence, the narrow angle of coverage achieved by this approach (and the associated reduction in nuisance alarms) comes at the expense of poor detection performance (and in particular, the existence of blind spots) in regions close to the lens of the detector.
Other approaches aimed at solving this problem may utilize multiple pyroelectric detectors and sophisticated electronic circuitry to achieve a narrow detection zone. However, these approaches have the drawback of increased cost and complexity of the detector.
There is thus a need in the art for an infrared detector that overcomes the aforementioned infirmities. In particular, there is a need in the art for an infrared detector which has a reasonable focal length and minimal degradation in detection performance close to the lens, and which avoids some of the cost and complexity inherent in some approaches to achieving a narrow detection angle detector. These and other needs may be addressed by the devices and methodologies described herein.